1. Field of the Invention
The present invention is related to coupled inductor structures, and more particularly, to a coupled inductor structure applied in multiple dielectric layers.
2. Description of Related Art
The multi-layer structure in such as an LTCC (low temperature co-fired ceramic) process gives designers great freedom in designing inductors, capacitors and transmission lines. However, inductors larger than 5 nH are not easy to implement by spiral or helix shapes due to size and self-resonant frequency limitations. In the paper “Design of miniature multilayer on-package integrated image-reject filters” written by Albert Sutono, Joy Laskar and W. R. Smith in the IEEE Trans. Microwave Theory and Tech., vol. MTT-51 part 1, pp. 156˜162. January 2003, a larger equivalent inductance value is achieved through a coupled inductor structure and a bandpass filter at 2.5 GHz is further implemented through the coupled inductor structure. In addition, in the paper “A Compact second-order LTCC bandpass filter with two finite transmission zeros” written by Lap Kun Yeung and Ke-Li Wu in the IEEE Trans. Microwave Theory and Tech., vol. MTT-51 No. 2, pp. 337˜341, February 2003, a bandpass filter is implemented through two parallel coupled inductors disposed at different layers with separation of about 100 μm.
Although inductance value can be increased through the structures proposed in the above-described papers, problem of misalignment between upper and lower layers occurring in stacking of layers in a LTTC process cannot be prevented. Typically, the misalignment is in the range of 20 μm˜50 μm, which can result in a dimension error of up to 20%˜50% and result in frequency deviation of the bandpass filter.
With reference to FIGS. 1a and 1b, a schematic representation of a conventional coupled inductor is shown. L1 and L2 represent self-inductance values of two separated inductors without any coupling. Nodes A and B respectively represent port 1 and port 2 of the conventional coupled inductor. When the two separated inductors are brought close to each other, magnetic coupling can occur. M denotes the mutual inductance value between the two separated inductors. The schematic of FIG. 1a can be transformed into an equivalent circuit of FIG. 1b, wherein inductance values LL1, LL2 and MM of FIG. 1b are calculated through following equations:
            LL      1        =                                        L            1                    ⁢                      L            2                          -                  M          2                                      L          2                -        M                        LL      2        =                                        L            1                    ⁢                      L            2                          -                  M          2                                      L          1                -        M                  MM    =                                        L            1                    ⁢                      L            2                          -                  M          2                    M      
By adjusting the mutual inductance value M between the two inductors, a desired larger equivalent inductance value MM can be obtained.
With reference to FIG. 2, a conventional multi-layer coupled inductor structure is shown, which achieves a larger equivalent inductance value based on the foregoing equations. As shown in FIG. 2, the multi-layer coupled inductor structure includes a first dielectric layer 10, a second dielectric layer 11, a top grounding layer 12 and a bottom grounding layer 13, wherein the first dielectric layer 10 has a substantially J-shaped metallic conductive wire 101 with a first opening 101a, the second dielectric layer 11 has a substantially J-shaped metallic conductive wire 111 with a second opening 111a facing a direction opposite to that of the first opening 101a of the conductive wire 101. The conductive wires 101, 111 respectively have self-inductance values L1, L2. Magnetic coupling occurs between two segment portions 101b, 111b of the conductive wires 101, 111 that are parallel to Y-axis and generates a mutual inductance value M. As described earlier, placing the two parallel segment portions 101b, 111b of the conductive wires 101, 111 at different layers does provide more freedom to designers in controlling the distance between them. However, the misalignment between the first and second dielectric layers 10, 11 can change the mutual inductance value between the conductive wires and further affect the equivalent inductance value MM. To be more specific, a straight-line coupled inductor has a size of about hundreds of micrometers in Y-axis, which is far more larger than size of the inductor in X-axis. As a result, the misalignment between the first dielectric layer 10 and the second dielectric layer 11 in X-axis has great influence on the coupled inductance.
U.S. Pat. No. 6,873,221 B2 with a title of “Multilayer Balun with High Process Tolerance” reduces degradation effects attributed to misalignment between layers by dividing a straight coupled line into two segments mutually vertical to one another. Although the technique adopted reduces the change of mutual inductance value, it does not offset the change in left-right displacement. Therefore, there still exists a problem that the mutual inductance value exceeds a permissible range due to too big change of the displacement.
Therefore, developing a technique to prevent horizontal deviation in the process from affecting inductor elements in different dielectric layers so as to prevent inductance value of the coupled inductor from exceeding a permissible range becomes critical.